At 400 grams he should be eating a large weaned(30-45 grams) or small rat(45-80 grams).
Start checking on craigslist or call around to some reptile stores if they don't carry them they may know where you can get them from.
Edit: I didn't realize he was eating frozen thawed so you could definitely order them online from someone like rodentpro.com. Shipping can be alittle expensive unless you are ordering in larger quantities.
Also, the mice should be fine for him until you are out.
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